1. Field of the Invention
This invention relates to a lamination type heat exchanger such as an evaporator including several tubes (refrigerant passages) formed by laminating metallic thin plates, which is suitable for a refrigerant evaporator of an automotive air conditioning apparatus.
2. Description of the Related Art
In recent years, in a refrigerant evaporator for an automotive air conditioning apparatus, inner fins are inserted into tubes (refrigerant passages) formed by laminating metallic plates so that a refrigerant side heat transfer area increases. This results in improvement of evaporator characteristics. When the inner fins respectively have a corrugated shape in cross-section, each of the refrigerant passages is divided into several straight-pipe like subpassages, and refrigerant flows independently in the respective subpassages from an inlet portion to an outlet portion without being mixed with the refrigerant flowing in the other subpassages.
The inventors of the present invention examined and studied this type of the evaporator, and found the following problems. First, if the refrigerant is unevenly distributed into the subpassages at the inlet portion, the unevenness of the distribution is kept in the subpassages, and may be encouraged in the subpassages by the following reason.
That is, under ordinal operational conditions of the evaporator, the liquid refrigerant expands to be gaseous refrigerant having a volume approximately 70 times as large as that of the liquid refrigerant so that the flow resistance increases. Therefore, when the gas region of the refrigerant is large in the inner fin subpassages, it become difficult for the refrigerant to flow in the subpassage. In addition, when a distribution amount of the refrigerant distributed into one of the subpassages is short relative to heat load on an air side, the refrigerant starts to evaporate at a refrigerant upstream side more than that in the other subpassages in which the distribution amount of the refrigerant is not short. As a result, the gas region is further increased to encourage the shortage of the refrigerant.
On the other hand, in the subpassage into which the refrigerant is distributed too much, the refrigerant starts to evaporate at a refrigerant downstream side more than that in the subpassage in which the refrigerant is short. Therefore, the gas region becomes relatively small, so that the refrigerant readily flows in the subpassage. This further encourages the excess of the refrigerant. In this way, the shortage and excess of the refrigerant distribution with respect to the air side heat load, which occurs when the evaporator starts, is further encouraged after the heat exchange between the refrigerant and air is carried out. In this case, as compared to a case (ideal state) where the evaporation of the refrigerant (heat exchange) is carried out evenly in every subpassages, the cooling capacity of the evaporator is lowered.
In addition, when the air flows from an air upstream side to an air downstream side in the heat exchanging part, the temperature of the air is gradually decreased. Therefore, an optimum distribution amount of the refrigerant into the subpassages on the air downstream side is inevitably smaller than that on the air upstream side. Therefore, when the refrigerant is evenly distributed into the subpassages, inevitably, the refrigerant becomes short in the subpassages on the air upstream side and becomes excessive in the subpassages on the air downstream side.